Joining yarns is a common operation, in particular in winding machines. It is also carried out on knitting machines, looms and on certain free-ended spinning frames. For a very long time this operation has been carried out by knotting the two yarn ends to be joined, so that a fault then appears in the fabric.
Methods have already been proposed for replacing the knots by invisible joints which as near as possible approach spinning. The main difficulty of such methods is obtaining sufficient tensile strength in the yarn. In the case of a knot the strength corresponds substantially to that of the yarn, whereas in the case of an invisible joint in which the yarns are twisted together, this strength cannot in practice be equal to that of the yarn. However, a method of this type can only be acceptable if the strength of the joint is at least 80% of the initial strength of the yarn.
Among the proposed methods, mention can be made of U.S. Pat. No. 3,903,680, in which the yarn pieces are untwisted, the fibers of one end are then separated to form a cone after which the other end is inserted into the open end and its fibers are in their turn spaced apart so that they penetrate between the fibres of the initially opened end. These two ends are then twisted together to recreate the yarn twist and connect together the two pieces. This method enables results to be obtained which can be good depending on the thickness of the yarns and the yarn material. However, with certain yarns, in particular relatively thick yarns formed from synthetic fibers, certain difficulties arise particularly because of the density of the fibres which makes their interpenetration difficult, and also because of the remaining twist in the fibers.
Pneumatic methods have also been proposed for intermingling the fibers of the two pieces to be joined. Thus U.S. Pat. Nos. 3,306,020, 3,407,583 and 3,487,618 propose different methods in which the untwisted ends of the yarns to be joined are placed in a chamber, and a turbulent air stream is created in order to intermingle the fibers of the two ends such as to form a zone in which the two pieces are joined. The appearance of such a joint does not resemble the appearance of the yarn, and the strength of such a joint is generally barely more than half the initial yarn strength.